Another vote for Trine

Also maybe have a look at Gorogoa. No gamepad support I think (click and drag). There's also a mobile version available.

Monkey Island games
Proteus
Dear Esther
Skyrim
Walking Dead series
Simcity
The Sims
Diablo 3

A third vote for Trine.

Are they available again?

AFAIK only on the Epic Store.

I think RiME checks all your boxes. Controls are very simple (there is controller support as well), you start playing immediately and the game slowly teaches you its gameplay mechanics. Some puzzles will make you stop and think, but none are too difficult to get permanently stuck. There are very few cinematics and, in fact, not a single line of dialogue throughout the whole game. It is also incredibly beautiful and the music is amazing. There is a story, but told in an unconventional and sensitive way. I consider it a true masterpiece.

The only downside is, depending on your mother's taste, she might not like the sudden change in one of the worlds/chapters, that goes from "explore at your own pace, relax and chill" to "run for your life and hide". The rest of the game is pretty peaceful.

Oh, and... you might want to wait for a sale.

They can still be found on PC on Epic Store but they can also be found on consoles, both retail and PSN.

Ghost of a Tale (tense!)
Monster Boy and the Cursed Kingdom (tense and people are saying it's quite hard, I agree)
Kynseed
Candleman (also tense)
Skyalr and Plux Adventures on Clover Island (not tense at all)
Knights and Bikes (controls are wonky, but maybe?)

I have one more recommmendation that I forgot before:

Brothers: A Tale of two Sons (available on GOG)

thank you, everyone! I do really appreciate your help, guys!. I'll carefully check those games :)

Also as far as platforming games go, Rayman 2 and Rayman Origins.

This!!!

Brothers is a wonderful game.

Granny in paradise

Google it.

yuck
was that necessary?

Super Meat Boy, or Geometry Dash

Both o these are strikingly simple in premise. Don't hit the spikes or saws or (in case of geometry dash) walls.

Just jump your way through levels in some intense platforming and platform running goodness.